In recent years, standardization for realizing broadband wireless Internet access targeting a transmission rate of 10 Mbps to 100 Mbps has been promoted and various kinds of technologies have been proposed. A requirement needed for realizing high speed transmission rate radio communication is to increase frequency utilization efficiency. Since the transmission rate and a bandwidth used are in a direct proportional relationship, a simple solution to increase the transmission rate is to broaden the frequency bandwidth to be used. However, frequency bands that can be used are becoming scarcer and it is therefore unlikely that sufficient bandwidth be assigned for constructing a new radio communication system. Consequently, it becomes necessary to increase frequency utilization efficiency. In addition, another requirement is to seamlessly provide services in a private area (isolated cell) such as a wireless LAN while realizing services in a communication area composed of cells such as mobile phones.
A technology that has a potential for meeting these requirements includes one-cell repetition OFDMA (Orthogonal Frequency Division Multiple Access). In this technology, communication is performed by using the same frequency band in all cells in a communication area composed of these cells, and a modulation system for performing communication is OFDM. This communication method can realize faster data communication while isolated cells have a radio interface common to that of a cell area as a matter of course.
An essential technology OFDM of the OFDMA will be described below. The OFDM system is used in IEEE802.11a, which is a 5 GHz-band radio system, and Digital Terrestrial Broadcasting. The OFDM system arranges several tens to several thousands of carriers at theoretically minimum frequency intervals with no interference for simultaneous communications. In the OFDM, these carriers are usually called subcarriers and each subcarrier is modulated by a digital system such as PSK (phase shift modulation) and QAM (quadrature amplitude modulation) for communication. Further, the OFDM is said to be a frequency-selective fading resistant modulation system in combination with an error correction system.
A circuit configuration for modulation and demodulation will be described using diagrams. Here, it is assumed that 768 subcarriers are used for the OFDM for a concrete description below.
FIG. 6 is a block diagram illustrating a schematic configuration of a modulation circuit of the OFDM. The modulation circuit shown in FIG. 6 includes an error correction coding part 501, a serial to parallel conversion part (S/P conversion part) 502, a mapping part 503, an IFFT part 504, a parallel to serial part (P/S conversion part) 505, a guard interval insertion part 506, a digital to analog conversion part (D/A conversion part) 507, a radio transmission part 508, and an antenna 509. Error correction encoding of information data to be transmitted is performed by the error correction coding part 501. If a modulation scheme of each carrier is QPSK (four-phase modulation), 2.times.768=1536 bits are output from an error correction coding circuit to generate one OFDM symbol. Then, 2 bits are input into the mapping part 503 at a time from the S/P conversion part 502 as 768-system data, and modulation is performed by the mapping part 503 for each carrier. Then, the IFFT part 504 performs IFFT (Inverse Fast Fourier Transform). The number of points of the IFFT usually used for generating a 768-subcarrier OFDM signal is 1024.
Data is allocated to f(n) (n is an integer between 0 and 1023) by the mapping part and thus the IFFT part 504 will output data t(n). Since only 768 pieces of data are input for 1024-point IFFT input in the present example, zero (both real and imaginary parts) is input as other pieces of data. Normally, f(0) and f(385) to f(639) correspond to input of zero. Then, after the data is converted to serial data by the P/S conversion part 505, guard intervals are inserted by the guard interval insertion part 506. Guard intervals are inserted for reducing interference between symbols when receiving an OFDM signal. If no guard interval is used, IFFT output t(n) is output in order of t(0), t(1), . . . , t(1023) and these form symbols of the OFDM. When guard intervals are used, a latter half part of IFFT output will be output in accordance with a guard interval length. If the guard interval length is ⅛ of a normal OFDM symbol, t(n) will be output in order of t(896), t(897), . . . , t(1023), t(0), t(1), . . . , t(1023). Then, after the data is converted to an analog signal by the D/A conversion part 507, the analog signal is converted to a frequency to be used for transmission, and then the data is transmitted from the antenna 509.
FIG. 7 illustrates a schematic view of spectrum of an OFDM signal after D/A conversion, a schematic view of time waveforms after D/A conversion, and a schematic view after frequency conversion of the spectrum to a transmission band. f(n) and t(n) in FIG. 7 are the same as those shown in the above description.
It is known that if, usually when transmitting or receiving an OFDM signal, the center of all bands is handled as DC in base-band processing, a sampling frequency of an A/D converter and D/A converter will be the smallest and also efficient. However, in the OFDM, as shown above, no data is usually allocated to a DC component, that is, a carrier corresponding to f(0). Thus, power of the DC component is also depicted as zero in FIG. 7. It is obviously theoretically possible to modulate the DC component, but the DC component is susceptible to noise (an influence of offset in the DC component of a circuit) in a transmitter or receiver and thus degradation of characteristics thereof is severe compared with other subcarriers. For this reason, almost all systems do not modulate the subcarrier of the DC component.
Japanese Patent Application Laid-Open No. Hei 10-276165 and Japanese Patent Application Laid-Open No. Hei 11-154925, for example, describe an influence of the DC offset and how to eliminate the DC offset.
FIG. 8 is a block diagram illustrating the schematic configuration of an OFDM demodulator circuit. Basically, an operation that is opposite to that performed by a transmission part is performed by a reception part. The demodulator circuit shown in FIG. 8 includes an error correction decoding part 701, a parallel to serial conversion part (P/S conversion part) 702, a propagation path estimation demapping part 703, an FFT part 704, a serial to parallel (S/P conversion part) 705, a guard interval (GI) removal part 706, an OFDM symbol synchronization part 707, an analog to digital conversion part (A/D conversion part) 708, a radio reception part 709, and an antenna 710. Frequencies of radio waves received by the antenna part 710 are converted down to frequency bands where A/D conversion can be performed by the radio reception part 709.
OFDM symbol synchronization of data converted to a digital signal by the A/D conversion part 708 is carried out by the OFDM symbol synchronization part 707. Symbol synchronization is to determine boundaries of OFDM symbols from continuously incoming data. Data whose symbol synchronization has been carried out is represented by t′(n). If there is neither multipath nor noise in communication at all, t′(n)=t(n) holds. Guard intervals are removed by the guard interval removal part 706. Therefore, after guard intervals are removed, t′(m) (m is an integer between 0 and 1023) will be extracted. Then, parallel conversion of the data into 1024 pieces of data is performed by the S/P conversion part 705. Then, 1024-point FFT (Fast Fourier Transform) is performed by the FFT part 704 before f′(m) is output to the propagation path estimation demapping part 703. However, since no modulation has been performed for m=0 and m=385 to 639 for transmission, f′(m) corresponding to such m are not input into the demapping part. Demodulation of subcarriers including propagation path estimation of 768 subcarriers is performed by the propagation path estimation demapping part 703. The data is converted to serial data by the P/S conversion part 702 and error corrections are carried out by the error correction decoding part 701 before demodulation of transmission data is completed.
Next, the OFDMA will be described based on the above OFDM. The OFDMA system forms two-dimensional channels on frequency and time axes, arranges slots for communication two-dimensionally in a frame, and allows a mobile station to access a base station using the slots. FIG. 9 is a diagram illustrating a two-dimensional frame configuration of the OFDMA. In this diagram, the vertical axis is the frequency axis and the horizontal axis is the time axis. One rectangle is a slot used for data transmission and a rectangle with oblique lines is a control slot used by the base station to transmit broadcast information to all mobile stations. This diagram indicates that one frame has nine slots in a time direction and twelve slots in a frequency direction, and 108 slots (among 108 slots, twelve slots are control slots) exist in total. Formally, a slot is represented by (Ta, Fb), with a time axis direction slot Ta (a is a natural number between 1 and 9) and a frequency axis direction slot Fb (b is a natural number between 1 and 12). A shaded slot in FIG. 9, for example, is represented by (T4, F7).
In the present specification, twelve slots configured in the frequency direction are called time channels and nine slots configured in the time direction are called frequency channels or sub-channels.
Subcarriers of the OFDM will be divided and allocated to the frequency channels. Since it is assumed that the OFDM has 768 subcarriers, 64 subcarriers are allocated to each channel if divided equally among twelve slots. Here, it is assumed that subcarriers are allocated in increasing order of spectrum in bands used for actual communication for convenience and thus subcarriers f640 to f703 are allocated to F1, subcarriers f704 to f767 to F2, . . . , subcarriers f960 to f1023 to F6, subcarriers f1 to f64 to F7, subcarriers f65 to f128 to F8, . . . , and subcarriers f321 to f384 to F12.
Communication from a base station (AP) to a mobile station (MT) will be considered. Many cases can be considered when the AP allocates data for 15 slots to the MT and it is assumed here that data is allocated to slots with vertical lines in FIG. 9. That is, data to be received by the MT will be allocated to (T2 to T4, F1), (T5 to T8, F4), and (T2 to T9, F11). It is also necessary to embed data indicating allocation of data in a control slot corresponding to the frequency to be used to indicate that the AP has allocated data to the MT. For the present example, (T1, F1), (T1, F4), and (T1, F11) correspond to such control slots.
The OFDMA system, based on what has been described above, allows a plurality of mobile stations to transmit and receive data to and from the base station by changing the frequencies and times. FIG. 9 illustrates a gap between slots for convenience, but whether or not there is a gap is not so important.
FIG. 10 is a block diagram illustrating a schematic configuration of a radio transmitter used for the OFDMA, and FIG. 11 is a block diagram illustrating the schematic configuration of a receiving circuit used for the OFDMA. A transmitting circuit shown in FIG. 10 has a data multiplexing part 901, and is divided into an error correction coding part 902, an S/P conversion part 903, and a mapping part 904 for the number of channels (one to twelve). An IFFT part 905, a P/S conversion part 906, a GI insertion part 907, a D/A conversion part 908, a radio transmission part 909, and an antenna 910 fulfill functions similar to those of the IFFT part 504, parallel to serial conversion part (P/S conversion part) 505, guard interval insertion part 506, digital to analog conversion part (D/A conversion part) 507, radio transmission part 508, and antenna 509 shown in FIG. 6 respectively.
In FIG. 10, the data multiplexing part 901 demultiplexes information data to be transmitted into twelve series in units of packets. That is, the multiplexer 901 physically specifies slots of the OFDMA specified by modules such as CPU (not shown). Then, error correction encoding is performed by the as many error correction coding parts 902 as the channels, the data is demultiplexed into 64-system data by the as many S/P conversion parts 903 as the channels, and modulation is performed by the as many mapping parts 904 as the channels for each carrier before IFFT processing is performed by the IFFT part 905. Operations thereafter are the same as those described with reference to FIG. 6.
A receiving circuit shown in FIG. 11 has a data multiplexing part 101, and is divided into an error correction coding part 102, a parallel to serial conversion part (P/S conversion part) 103, and a propagation path estimation demapping part 104 for the number of channels (one to twelve). An FFT part 106, a GI removal part 107, a synchronization part 108, an A/D conversion part 109, a radio receiving part 110, and an antenna part 111 fulfill functions similar to those of the FFT part 704, serial to parallel conversion part (S/P conversion part) 705, guard interval (GI) removal part 706, OFDM symbol synchronization part 707, analog to digital conversion part (A/D conversion part) 708, radio reception part 709, and antenna 710. Similar to the receiving circuit shown in FIG. 8, FFT processing is performed for received radio waves, and each of the twelve series of data undergoes propagation path estimation, demapping, and error correction processing before being input into the data multiplexing part 101. Information data is processed by the data multiplexing part 101 before being output.
Modulation and demodulation processing shown here is only an example. Particularly, as many blocks as the number of channels, that is, twelve blocks are shown, but the present invention is not limited to this number. Japanese Patent Application Laid-Open No. Hei 11-346203 described a basic configuration of an OFDMA transmission apparatus.    Japanese Patent Application Laid-Open No. 10-276165    Japanese Patent Application Laid-Open No. 11-154925    Japanese Patent Application Laid-Open No. 11-346203